Miche Braden gives "Sweet Child o' Mine" a fabulous bluesy, jazzy, New Orleans Style treatment here. I love the instrumentation with the brass and the clarinet:
I have a huge girl-crush on Sheryl Crow and she nails this cover:
I usually find Gregorian chants very soothing, but this one takes the cake for the most unlikely arrangement of the song:
For the complete polar opposite of Gregorian chant, and, I think, the least recognizable version of the song, here's the electronica cover by Akasha.
And finally, here's a Brazilian bossa band with a sweet, mellow arrangement. Extra points for the use of the flute on the famous guitar solo riff.